This invention relates to charged particle beam processing of semiconductor wafers in vacuum and, more particularly, to a method for pretreatment of a masking layer adhered to the surface of the semiconductor wafer prior to implantation of dopant ions.
Ion implantation has become a standard technique for introducing impurities into semiconductor wafers in a controlled and rapid manner. A beam of ions is generated in a source and directed with varying degrees of acceleration toward the semiconductor wafer. The impurities are introduced into the bulk of semiconductor wafers by using the momentum of the ions as a means of embedding them in the crystalline lattice of the semiconductor material. Maintaining uniformity of impurity concentration over the surface of the semiconductor wafer and controlling total impurity dosage are of utmost importance in semiconductor processing. In addition, one of the major objectives in commercial semiconductor processing is to achieve a high throughput in terms of wafers processed per unit time.
Prior to the ion implantation process, each semiconductor wafer is typically coated with a masking layer such as a photoresist layer. A desired pattern to be ion implanted is produced in the photoresist layer by conventional photolithographic techniques. The photoresist layer is removed in areas where ion implantation is to take place and remains as a mask over the remainder of the wafer surface.
During ion implantation, the ion beam is scanned over the surface of the wafer by electrostatic, magnetic or mechanical means, or by a combination thereof. In the regions where the photoresist layer has been removed, the ions penetrate into the bulk of the semiconductor material and produce desired doping characteristics. In the regions where the photoresist layer remains on the surface of the wafer, the ions penetrate the photoresist and undergo collisions with electrons and nuclei of the photoresist material and eventually come to rest. Since the photoresist material is usually an organic polymer, the energetic ions cause scission of hydrocarbon chains as they travel through the resist material. As a result, outgassing of hydrogen, water vapor and other materials from the surface of the photoresist layer occurs, and at least the outer portion of the photoresist layer is carbonized.
As is well known, ion implantation is performed in a vacuum, wherein the pressure is preferably maintained at or below 1.times.10.sup.-5 Torr. When the above-described outgassing of materials from the photoresist occurs, the pressure level in the processing chamber increases. The increase in pressure in the processing chamber produces errors in the measured dose due to collisions between the ion beam and the gas molecules outside the Faraday measuring system. When these collisions occur, some of the ions in the ion beam are neutralized. Since the Faraday system registers dopant atoms only if they carry an electrical charge, the Faraday system is not able to measure the neutralized portion of the ion beam, and a dose error is introduced. As noted above, such dose errors are unacceptable, particularly in the fabrication of LSI or VLSI integrated circuits, where device characteristics can be substantially altered by dose errors.
It has been observed that the peak value of the pressure increase, or pressure burst, caused by photoresist outgassing increases as the ion beam current is increased. As the peak value of the pressure burst increases, the potential dose error is similarly increased. Thus, attempts to improve the throughput of ion implantation systems by increasing beam current have been frustrated by photoresist outgassing. For lower ion beam currents, the pressure burst caused by photoresist outgassing has a lower peak value but is longer in duration. Ion implantation has, therefore, often been performed in a two-step process to keep the above-described dose errors within acceptable limits. First, a relatively low current implant of the dopant ions is performed until photoresist outgassing is substantially completed. Then a high current implant of the same dopant ions is used to achieve the desired dose. While such an approach keeps dose errors within acceptable limits, throughput is reduced significantly by the first low current step of the process.
It is therefore an object of the present invention to provide a novel method for increasing the throughput of ion implantation systems.
It is another object of the present invention to provide a method for ion implantation wherein the dose errors caused by photoresist outgassing are substantially reduced without significantly reducing system throughput.
It is still another object of the present invention to provide a method for pretreatment of a masking layer prior to charged particle beam processing in vacuum.